The mechanism of replication of telomeres, which are the end structures of linear chromosomes, is currently an important and fundamental biological problem. There is evidence that telomere replication occurs via a common mechanism in all known organisms, from protozoa to humans. Telomeric DNA is known to adopt novel structures that may play a role in this process. The model system to be used in the present proposal are the telomeric DNA sequences from the ciliated protozoan Oxytricha. These sequences were chosen over the sequences from other organisms because our collective knowledge about telomeres, and the proteins that interact with telomeres, is greater for no other organism. The specific purpose of the research in this proposal is to understand the structures adopted by telomeric DNAs from Oxytricha. The structural studies are centered on testing the G-quartet model for telomeric DNA structure. First, the details of the structure of telomeric DNA will be investigated using a combination of base analog substitution experiments with high resolution proton NMR spectroscopy. Second, the thermodynamics and ion binding properties of the various telomeric forms will be addressed by thermal denaturation studies using UV-absorption spectroscopy and circular dichroism. Ultimately, these studies will be extended to similar telomere sequences from other organisms.